Field of the Invention
The invention relates to a method and a device for regulating the temperature in a laser-operated printing plate imaging unit of a printing press, particularly an offset printing press.
For the imaging or image formation of printing plates for printing presses, digitally operated imaging or image forming units are increasingly used presently in addition to the conventional method of film exposure; the imaging units receive the image data in the form of digital bit-patterns generated in the pre-press system and transfer them to the printing plate. For this purpose, the imaging units possess a light source, and the light from the light source is focused on a respective location of the printing plate through an optical lens system, the light source being switched on or off, depending upon whether or not a pixel is to be produced at the respective location.
U.S. Pat. No. 5,351,617 discloses a laser-operated imaging unit for a printing plate provided with a special coating and mounted on the plate cylinder of an offset printing press, the laser light of the imaging unit being generated through a laser diode unit and being subsequently conducted via an optical light-guiding cable to an optical focusing unit arranged near the plate cylinder, the focusing unit being moved in a motorized manner across the surface of the plate cylinder, in parallel with the longitudinal axis of the plate cylinder, and the laser light being focused on the respective locations on the printing plate. By rotating the plate cylinder accordingly, imaging is performed on the entire surface of the printing plate mounted on the cylinder.
The aforementioned U.S. Pat. No. 5,351,617 furthermore shows a device, by which multiple optical focusing units connected to respective laser-light sources by optical light-guiding cables are moved across a flat printing plate and illuminate or expose it at the respective location.
With the aforedescribed imaging units operated by laser diodes, a problem exists in that the intensity of the laser light is greatly influenced by the temperature of the respective laser-light source, in this case a laser diode. Due to the conventional substantially exponential temperature-dependency of the intensity of the generated laser light, temperature variations from 0.5.degree. C. to 2.degree. C., in the case of a laser diode, already have such a disadvantageous effect upon the imaging results, that the quality deficiencies in the finished printed image caused thereby can easily be noticed by the human eye.
The quality deficiencies result due to a varying light intensity of the laser light caused by the temperature of the respective laser diode being too high or too low, the pixels to be produced on the printing plate vary greatly, so that the printed image created with the printing plate shows defects leading to the aforedescribed noticeable quality deficiencies.
With the imaging of a printing plate, the temperature variations of the laser diodes are particularly caused by the fact that the laser diodes, in the switched-on state thereof, convert a great part of the electrical energy fed thereto into joulean heat, and in the switched-off state thereof, i.e., in the regions where no imaging takes place, the laser diodes do not generate any heat. In practice, the quality deficiencies occur especially in those areas of the printing plate where pixels are produced by the respective laser diode unit only sporadically, because the laser diode unit, having been switched off for a very long period of time, has cooled off and, when switched on again, generates a reduced light intensity because of the required heating-up time.